At 3:08 a.m. local time, a M=5.4 earthquake struck northern Peru within the eastern foothills of the Andes. This region of Peru is not densely populated, meaning not many people were exposed to shaking. Additionally, because the quake occurred at a depth of 38 km (according to the USGS), only light shaking was reported, meaning the quake likely caused no disruption. Even though the USGS does not have a focal mechanism, based on the quake’s location within the Northern Andes, it likely occurred on a thrust fault. Even though this earthquake was not significant, it did occur in a part of South America where the seismic potential is different than most other places along the western margin of the continent.

This Temblor map shows the location of today’s M=5.4 earthquake in the Peruvian Andes.

First, it should be pointed out that smaller magnitude earthquakes are frequent within the foothills of the Andes around the location of today’s event. These quakes are a product of the South American continent being compressed due to subduction of the Nazca Plate is beneath the South American continent. In addition to these, much larger earthquakes along the subduction zone are also quite common. In fact, three of the ten largest quakes worldwide since 1900 have occurred along the western margin of South America. However, in northern Peru, large earthquakes are almost non-existent.

In the Northern Andes (Peru, Ecuador, and Colombia), plate convergence is approximately 60 mm/yr. While this number does not vary much to the north or south, the way strain is accommodated is. In the figure below from Nocquet et al., 2014, large subduction zone earthquakes since 1900 are shown. What is apparent from this map is that outside of the M=7.6 in 1960, northern Peru and southern Ecuador have seen no recent large earthquakes. The question then becomes, why?

This figure from Nocquet et al., 2014 shows the location of large magnitude earthquakes in South America since 1900. Today’s earthquake is marked by the yellow star. What is evident here is the absence of large earthquakes in northern Peru and southern Ecuador. In this figure the black cross-hatching represents rupture zones from the 1906 and 1746 earthquakes.

By using GPS stations scattered around the country, scientists have determined that in northern Peru, there isn’t much ongoing deformation of the continent. In contrast, the areas north and south show significant deformation. What this suggests is that in northern Peru, the subduction zone is freely slipping. This means that that not as much stress can accumulate, and large megathrust earthquakes are not common. However, north and south, where the subduction zone can lock, large quakes can occur.

While large magnitude earthquakes are not likely in northern Peru, this does not mean they are impossible. Based on recent GPS measurements analyzed by Villegas-Lanza et al., 2016, a M=8.6-9.0 could occur in northern Peru about every 1,000 years. This would give it a similar recurrence to the great 2011 M=9.0 Tohoku, Japan earthquake. Nonetheless, the seismic potential of the northern Peru subduction zone is less than most other places along western South America.

By using the Global Earthquake Activity Rate (GEAR) model, which is available in Temblor, we can see this. The GEAR model uses global strain rates and seismicity since 1977 to forecast the likely earthquake magnitude in your lifetime anywhere on earth. As can be seen, in northern Peru the largest likely earthquake is approximately M=7.0, while to the north and south, larger, potentially more damaging quakes can occur. To view the GEAR model, click here.

This Temblor map (with faults removed) shows the Global Earthquake Activity Rate (GEAR) model for the western margin of South America. What is evident from this map is that in northern Peru, larger magnitude earthquakes are not as likely as they are to the north and south. This is likely due to the fact that the subduction zone is freely slipping as opposed to being locked. Additionally, within Peru, there are 2 small lobes to the north and south where M=6.25+ earthquakes are possible. This magnitude is larger than what is expected in most others in inland Peru.